Mortality within the first 24 hours after stroke frequently results from cerebral edema for which no effective treatment is available. Injury to endothelium by oxygen radicals in a well- recognized cause of edema formation in the lung and other organs but its role in cerebral ischemia is poorly defined. We show that enzymatic oxygen radical scavengers can reduce infarct volume in a rat focal stroke model and hypothesize that xanthine oxidase (XO) is an important source of these oxygen radicals. Evidence is presented to show that cerebral ischemia induces the conversion of a normally benign enzyme, xanthine dehydrogenase (XDH), to a free radical-producing oxidase which reduces molecular oxygen to the toxic intermediates, superoxide and H2O2. Most of the brain XDH and XO is localized within the endothelium, putting the blood-brain barrier at risk of oxygen radical injury. In this proposal, the role of cytotoxic oxygen free radicals will be investigated in early post-ischemic changes in cerebral blood flow, edema formation, neutrophil influx and infarction in a focal stroke model in the rat. The stroke is induced by occluding the right middle cerebral artery (MCA) and transiently clamping both carotid arteries for up to 90 minutes, which yields a reproducible, quantifiable infarct restricted to the right cortex and ideal for examining pharmacological efficacy. The effects of treatment with specific enzymatic radical scavengers on the development of cytotoxic and vasogenic edema, changes in cerebral blood flow and histologic injury after the MCA-carotid occlusion will be evaluated. To optimize delivery to sites of free radical production, the oxygen radical scavenging enzymes, superoxide dismutase and catalase, will be either conjugated to polyethylene glycol or entrapped in liposomes, which increases their circulatory half-life and cell entry. The contribution of ischemia-induced XDH conversion of XO to injury will be examined by depleting XDH/XO activity in rats by three independent methods and measuring the effects on infarct volume, edema formation, cerebral blood flow and histologic injury after the MCA-carotid procedure. Rates of oxygen radical production in vivo will be measured after treatment with free radical scavengers and xanthine oxidase inhibitors. These studies will help define the role of oxygen radicals in cerebral ischemia and may lead to new therapeutic approaches in the treatment of stroke.